This invention relates generally to telecommunication systems employing A/D and D/A converters and more particularly, it relates to a dual channel A/D and D/A converter for use in a dual subscriber line audioprocessing circuit (OSLAC) which performs the functions of converting an analog voice signal into digital PCM samples and converting digital PCM samples back into an analog signal as well as the filtering functions, all associated with the four-wire section of the subscriber line circuitry in a digital switch.
In view of the large number of analog/digital interfaces used in telecommunication systems, there has been a great demand for developing low cost integrated circuits to perform the A/D and D/A functions in the subscriber loop. One of the prior art techniques employs an oversampled multi-channel coder which incorporates four A/D converters with transmit low pass filters on one single chip. This prior art technique is disclosed in a research paper written by Bosco Leung entitled "Multi-Channel TCM A/D Interface Using Oversampling Techniques." It is generally known that A/D converters which have a single ended structure are sensitive to crosstalk from other circuits on a chip. Since there are four converters used on the same chip, there will exist a certain amount of crosstalk between the different channels. Thus, in order to overcome this crosstalk problem the research paper to Leung utilizes a fully differential architecture consisting of a MOS switched-capacitor integrator. The chip with four channels was reported to have a crosstalk of -83 db.
However, the use of switched-capacitor techniques for constructing such A/D converters suffers from the limitation of exposing the switching node to a high impedance state and to clock feed-through digital noise. Since the high impedance node takes a long time to settle down, this may in turn cause crosstalk errors and undesirable noise. Further, this approach has the disadvantage of requiring the use of precisely matched capacitances. Therefore, it has been counted in the fabrication of such switched-capacitor integrator devices on a chip using CMOS technology the need of performing an additional masking step which increases manufacturing costs.
The dual channel A/D and D/A converter of the present invention operates in a differential mode and provides an improved crosstalk isolation by a factor of 7 between the two channels without the use of any switched-capacitor technique. The dual channel converter of the present invention is provided as a part of a dual subscriber line audio-processing circuit (DSLAC) manufactured and sold by AMD, Inc., Sunnyvale, Cal., part No. Am 79CO2/A. The DSLAC is an integrated circuit formed of a single-chip package which integrates the key factors of an analog linecard into one programmable, high performance dual CODEC-filter device for two telephone lines via a transformer or a subscriber line interface circuit (SLIC).